US President George W Bush has said a peace treaty with North Korea can be achieved once Pyongyang ends its nuclear weapons programme.

He restated his position during an odd exchange with his S Korean counterpart who asked him to be "clearer" on the issue during a joint news conference.

Earlier, Mr Bush called for the strengthening of democracy across Asia, singling out Burma for criticism.

He was speaking in Sydney ahead of a summit of Asia-Pacific leaders.

Tense exchange

Following talks with South Korean President Roh Moo-hyun on the margins of the summit, Mr Bush spoke of progress on the issue of North Korea's nuclear programme.

Then, in an unusual departure from the normal pleasantries at such events, Mr Roh said: "I might be wrong. I think I did not hear President Bush mention the declaration to end the Korean War just now. Did you say so, President Bush?"

We seek an Asia-Pacific region that is growing in freedom and prosperity and peace

Mr Bush responded by saying a peace treaty was in the hands of North Korean leader Kim Jong-il, and whether he gave up his nuclear weapons.

"If you could be a little bit clearer in your message, that would be very much appreciated," Mr Roh pressed.

"I can't make it any more clear, Mr President. We look forward to the day when we can end the Korean War. That will happen when Kim Jong-il verifiably gets rid of his weapons programmes and his weapons," Mr Bush said, bringing the exchange to an end.

The two Koreas are still technically at war, having never signed a peace treaty after the conflict ended in 1953. The US signed an armistice but not a peace deal.

South Korea has sought to engage with its impoverished northern neighbour in recent years under a so-called "sunshine policy".

Witnesses said that although the tone of the two leaders' conversation remained light, Mr Bush noticeably tensed during the exchange.

This may have been a reflection of the pressures he faces over North Korea back home, the BBC's diplomatic correspondent Jonathan Marcus says.

There are those within the Bush administration that are uneasy at the recent openness towards North Korea following its moves to declare and disable its nuclear programme, and remain distrustful of Pyongyang's intentions, he adds.

In a separate development on Friday, US chief negotiator to the nuclear talks, Christopher Hill, said experts from the US, China and Russia would travel to North Korea next week to carry out a survey of nuclear facilities to be disabled.

Burma criticism

Earlier, Mr Bush gave a wide-ranging speech to business leaders in Sydney in which he said free countries should work together to support democratic institutions.

"We seek an Asia-Pacific region that is growing in freedom and prosperity and peace," he said.

He singled out Burma, calling on the military government to free all political prisoners, including opposition leader Aung San Suu Kyi.

And he urged China to use next year's Olympics to show "greater openness".

Mr Bush also called for a break to the deadlock in the Doha round of trade talks.

He said the round was a "once-in-a-generation" chance and that America was prepared to show flexibility.

"The United States is committed to seizing this opportunity and we need partners in this region to help lead the effort," Mr Bush told the business leaders at Sydney's Opera House.

He also appealed for co-operation on climate change and said China and India had to be part of the process.